Nov. 11, 2005
Special to World Science
A collision between two galaxies 100 million years ago may have led them to spit out a colossal black hole that’s still soaring through space,
some astronomers have calculated.

If some astronomers are correct, the bright
object at the lower right is a black hole on a feeding frenzy, millions of
years after being kicked out of the galaxy at the upper left. (Hubble
Space Telescope)

If correct, the proposal would be the first evidence of a possibility astrophysicists have theorized for years: a black hole’s expulsion from a galaxy.

Indirectly, it could also shed light on how some black holes became as big as they are—a longstanding puzzle that’s also entangled with the
question of how galaxies formed.

A team of researchers describe the results in a paper to appear in an upcoming issue of the
Monthly Notices of the Royal Astronomical Society, a research journal.

They claim an enigmatic object known as HE0450-2958, estimated to weigh as much as 400 million suns or more, may be the expelled black hole.

HE0450-2958 is designated as a quasar, one of a class of objects that has puzzled scientists for decades. Quasars shine stupendous amounts of light across huge distances, some from practically the edge of the known universe.

What they are is uncertain, but most scientists think they’re probably gargantuan black holes, objects consisting of vast amounts of matter crammed into a small space. Black holes have overpowering gravity that drags in anything nearby with such force that the sucked-in objects are shredded and heated, so that they radiate intense light.

Quasars are thought to sit at the centers of big galaxies. This is because most galaxies
seem to harbor giant black holes at their centers, so this seems a logical place for a quasar. Quasars would
shine so brightly because they’re eating the stars and other material plentifully available in the surrounding galaxy.

But HE0450-2958, estimated to lie more than 3 billion light-years away from us—a light-year is the distance light travels in a year—seems to have no large home galaxy. This has puzzled astronomers, because without this, it should have little to “eat” and thus shouldn’t be shining brightly.

The new research suggests HE0450-2958 is an expelled black hole that still enjoys a quasar-like diet, possibly because it dragged enough material along with itself during its ejection to do so.

The scenario fits with calculations suggesting such events should occasionally happen when two galaxies collide, the researchers noted.

Moreover, the merged galaxy that they identify as black hole’s original home is still nearby, a paltry 20,000 light years away. It’s shining strongly with what seems to be widespread
star formation, an expected byproduct of a merger.

“Each of the merging galaxies is expected to have contained at least one supermassive black hole,” explained Martin G. Haehnelt of the University of Cambridge in Cambridge, U.K, and colleagues in the paper.

The expelled hole would have been one of those monoliths. Haehnelt’s team calculated it has been speeding away from the home galaxies at
some 300 kilometers (180 miles) per second on average, more than 100 times faster than the fastest
aircraft.

Exactly how it would have been coughed out is complicated, but two existing theories explain how it might happen.

In one, two galaxies merge, one of which already contains two huge black holes thanks to a previous merger. The new merger puts three supermassive black holes close together. They start orbiting each other because of their gravity.

But three objects don’t orbit each other very easily. They do a complex dance that often leads to violent rejection for the smallest one, due to
what astronomers call a “gravitational slingshot” from the other two.

In the other scenario, each merging galaxy has one supermassive black hole. These
eventually crash together, emitting a blast of a peculiar form of radiation called gravitational waves, whose existence Einstein postulated. If these waves burst out more strongly from one side of the merged hole than the other, their lopsided force might kick
it out of the galaxy.

Either way, the results could shed light on the formation of black holes—and through that the
evolution of galaxies, Haehnelt said.

Astrophysicists have trouble explaining how black holes got as big as they apparently did,
very early in cosmic history. Two competing views suggest the holes either grew through repeated mergers, or by sucking in whatever was around them.

There’s a related problem in explaining how galaxies became so big, so early in the history of the universe. Since black holes and galaxies seem to be intimately linked, astrophysicists
suspect the problems are related.

Haehnelt proposes that if the speeding quasar results from a black hole merger, it ironically suggests mergers weren’t a major way for black holes to grow, as the merger would
shoot them into deep space— away from other black holes with which to merge.

Separately from Haehnelt’s group, other researchers have also been studying whether HE0450-2958 is a rejected black hole. Some
agree it is. Others don’t.

David Merritt of the Rochester Institute of Technology in Rochester, N.Y., posted a paper on the Internet Friday saying the object is surrounded by too much gas to be an ejected black hole. It would have lost that gas during the ejection, he
wrote.

If it is a thrown-out hole, its exile might not be
permanent, Haehnelt and colleagues contended: the far-reaching gravity of the merged galaxy may well be strong enough to eventually drag
it home.